How to use a texture in real world scale?
There are multiple choices of texture spaces you can use in Blender. 



If you decided to use Object space instead of the UV space, you would have some control over it in the Object Data Properties panel in the Properties Editor: 



However with UVs typical thing to do to control the scale of the texture would be to use a Vector Mapping node in the material as you see in the example or to scale the UVs themselves. UV space can and often must be warped and shaped in many ways that are impossible to match to the actual scale of the object so if you want a UV mapped texture or parts of it to match the real size you have to make sure that happens yourself. 

You could make yourself a plane of the same size you have the texture, place the texture on it and then match the scale of it on the object visually or you could project the UVs in a specific way so you know that what you project is some specific length compared to your texture and maintain the ratio by scaling the UVs by that ratio... there are probably hundreds different situations and hundreds of ways of doing this so you just have to come up with one that suits your situation best.